Rotordynamics of Energy Storage Flywheels

One of the attractive design configurations for energy storage flywheels is one or more wheels hung at the bottom of a vertical shaft, with one or two bearings at the top of the shaft. In the terminology of rotating machinery engineers, the rotor is said to be "overhung" since there is no bearing outboard of the wheel. At the top, the shaft can be coupled to a motor/generator unit, which may or may not be included inside the vacuum chamber with the flywheel. All or part of the gravity thrust load can be supported with a friction-free magnetic bearing. This design has many advantages, but there have been repeated instances since the 1970's of rotordynamic instability problems associated with flywheels built with this configuration. Another application of this design configuration is the test apparatus used for spin testing of flywheels and turbomachinery wheels, where the wheel is suspended from the air turbine driver by a quill shaft. This application has also been occasionally subject to rotordynamic instability problems.

Rotordynamic instability is characterized by orbital whirling of the wheel at its lowest natural frequency even while the rotor speed is much faster. It is not significantly influenced by rotor balance, and can become violent with very large and destructive amplitudes if not suppressed by proper design. In energy storage flywheels, whirl instability is typically induced by internal damping (hysteresis) in the rotating assembly. Collaborating with Dr. Richard Schneider, Dr. Vance built a flywheel in the 1970's made of woven high-strength fibers, but with no bonding agent. Instead of a steel shaft, the wheel was suspended from wound ropes that allowed the critical speed inversion to take place at very low rotor speeds. Consequently the flywheel was self-balancing, to the extent that no balance masses need to be added for smooth synchronous operation at high speeds. However, the wheel whirled at its very low natural frequency (which could easily be tracked by the human eye) as a result of the internal friction in the rotating support system. Dr. Vance solved the whirl instability problem by suspending the motor from an anisotropic gimballedsupport. This design will also work to stabilize flywheels of conventional design, as Dr. Vance demonstrated with a laboratory model and a computer simulation. The extension to spin test hardware is straightforward, provided that the air turbine driver can be properly supported as described in the reference below.

Reference

.Vance, J.M., "Design for Rotordynamic Stability of Vertical-Shaft Energy Storage Flywheels", AIAA 2004-5605, Proceedings of the 2nd Energy Conversion Engineering Conference, August 16-19, 2004, Providence, Rhode Island.